Doctors examined the girl and surgically removed a 2-inch-long growth they later discovered was an embryo.
"They could see on the right side that her belly was swollen, but they didn't suspect that this tumor would hide an embryo," said Iakovos Brouskelis, director of Larissa General Hospital.
The girl has made a full recovery, he said.
The embryo was a formed fetus with a head, hair and eyes but no brain or umbilical cord, said Dr. Andreas Markou, head of the pediatric department.
He said cases in which one twin absorbs the other in the womb occurs in one in every 500,000 live births.
